Add telegram parse_mode option

[polshe-v]

Description

Added more parsing modes for Telegram and made separate option for setting parse_mode in alert rule. In order to make alert_text look pretty in all modes I added conditional alert_text formatting for markdown and html.

Checklist

• I have reviewed the contributing guidelines.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• I have updated the documentation.

Comments

There was a problem running make test-docker:

py310 run-test: commands[1] | flake8 --config ../setup.cfg .
./loaders_test.py:85:8: E275 missing whitespace after keyword
./loaders_test.py:86:8: E275 missing whitespace after keyword
./loaders_test.py:112:8: E275 missing whitespace after keyword
./loaders_test.py:113:8: E275 missing whitespace after keyword
./loaders_test.py:138:8: E275 missing whitespace after keyword
./loaders_test.py:139:8: E275 missing whitespace after keyword
ERROR: InvocationError for command /home/elastalert/tests/.tox/py310/bin/flake8 --config ../setup.cfg . (exited with code 1)

I added whitespace after del in these strings and all tests worked fine, all passed.

[jertel]

We typically require all contribution checklist items be completed, but since your PR identified some new, unrelated linting errors, I completed the remaining requirements for you:

• Added unit test coverage for new code
• Added CHANGELOG.md entry
• Clarified the new setting's documentation
• Fixed unrelated lint errors

Please ensure future PRs have all contribution requirements completed, or leave the PR in a draft mode until it's ready to be reviewed.

Thanks for your contribution!